Septum pellucidum, velum interpositum
Septum pellucidum is a thin, triangular, vertical double membrane separating the anterior horns of the left and right lateral ventricles of the brain The septum pellucidum is located in the midline of the brain It is attached superiorly to the corpus callosum and posteriorly to the anterior part of the fornix Cavum septum pellucidum (CSP) is a normal variant CSF space between the leaflets of the septum pellucidum. It is sometimes called the fifth ventricle. CSP is present in the normal fetus, but over 85% of them fuse by 3-6 months of age. It is an elongated finger shaped CSF collection between lateral ventricles Location: between frontal horns of the lateral ventricle Relations: * anterior: genu of the corpus callosum * superior: body of the corpus callosum * posterior: anterior limb and pillars of the fornix * inferior: anterior commissure and the rostrum of the corpus callosum * lateral: leaflets of the septum pellucidum The cavum vergae (CV), along with the cavum septum pellucidum (CSP) is a persistence of the embryological fluid-filled space between the leaflets of the septum pellucidum and is a common anatomical variant. The CV is sometimes referred to as the 6th ventricle. The CV is the posterior extension of the CSP, between the fornix. posterior to the anterior columns of thefornix, lying inferior to the splenium of the corpus callosum, but it may exist independently 1-3. Relations: * anteriorly - fornix * posteriorly - splenium of the corpus callosum * superiorly - body of the corpus callosum * inferiorly - transverse fornix The CSP commonly occurs, with or without cavum vergae. During development, these spaces obliterate postero-anteriorly - the cavum vergae followed by the cavum septum pellucidum. The velum interpositum is a small membrane containing a potential space just above and anterior to the pineal gland which can become enlarged to form a cavum velum interpositum. The velum interpositum is formed by an invagination of pia matter forming a triangular membrane at the apex . Boundaries * superiorly - the columns of the fornices and hippocampal commisure (psalterium) reaching as far forward as the foramen of Monro. * inferiorly - the internal cerebral veins and tela choroidea of the third ventricle * inferolaterally - the thalamus. * posteriorly- the narrow base of the triangle abuts the splenium of the corpus callosum 1 Varies in shape, sometimes interposed between the internal cerebral veins and splenium, and depending on whether or not there is a cavum vergae (in which case the columns of the fornices are displaced inferiorly, narrowing the velum interpositum). The space between the two leaves of pia contains the posterior medial choroidal arteries. A cavum velum interpositum (CVI) is a normal variation where there is a dilated CSF space involving the velum interpositum. * If > 1cm in axial transverse measurement, with mass effect, it is termed cyst of the velum interpositum. * Triangular CSF space * Location ** between lateral ventricles ** anterior extension to foramen of monro ** posterior extension from splenium of corpus callosum and fornix * Splays the columns of the fornix and flattens internal cerebral artery below